1. Field of the Invention
This invention relates to a noise reducing system, and more particularly to improvements in a system which reduces noise in television signals by the use of a frame memory.
2. Description of the Prior Art
In order to reduce noise included in television signals, there has been proposed a system using a frame memory, wherein a difference signal between an input television signal and a television signal representing the preceding frame of the television picture as stored in the frame memory is generated; depending upon the magnitude of the difference signal, the mixing ratio between the input television signal and the signal obtained from the frame memory is varied; and a part of the mixed signal is derived as a signal with the noise reduced, while the other part is stored into the frame memory; whereby the noise included in the input signal is reduced. (SMPTE Journal, Vol. 87, No. 3, March '78, "A Digital Noise Reducer for Encoded NTSC Signal" by R. H. McMann, S. Kreinik, J. K. Moore, A. Kaiser and J. Rossi)
As will be explained in more detail later, the prior-art noise reducing system proposed has a high effect of reducing noise for pictures of small variations. However, in a picture area where the frame difference signal is very large, the noise reducing effect is low and the signal-to-noise ratio or S/N ratio is inferior, the quality of the reproduced picture becomes unnatural, and a very high effect of reducing noise cannot be demonstrated.